As the precision of analog-to-digital converters increases, the processing problems and circuit design problems also increase due to the very low voltage increments that must be sensed. Additionally, the linearity of conventional analog-to-digital converters is being more tightly specified by consumers. In order to meet these specifications, it is necessary to fine tune both the processing and the circuit design in order to achieve the desired linearity.
One component that contributes to non-linearity problems is the capacitor in a switched capacitor analog modulator on the analog-to-digital converter. Typically, this capacitor is utilized to sample an input voltage by charging up the capacitor to the input voltage and then redistributing this charge to the input of an integration or comparator circuit. As the voltage on the input changes, the value of the capacitance also changes, resulting in an inherent non-linearity in the analog-to-digital converter. For high precision analog-to-digital converters, this can be a problem.
In a capacitor, the charge at the plate-to-dielectric interface is modulated by the applied voltage. Due to the finite volume and charge density in some type of plates, a depletion region can be formed in either or both of plate-to-dielectric interfaces, the width of which varies with voltage. Depending upon the type of material that the plate is fabricated from, this depletion region can significantly impact the voltage characteristics of the capacitor. For example, in double poly capacitors in which two polycrystalline silicon plates are provided with a silicon dioxide dielectric, the variation in capacitance can be substantial if the doping differs significantly between the plates.
With respect to Metal Oxide Silicon (MOS) or Silicon Oxide Slicon (SOS) capacitors, which are the type normally incorporated into integrated circuits, the nominal capacitance value and the rate of change of capacitance over some voltage interval is utilized to specify the capacitor. This is defined as the voltage coefficient of capacitance which is the rate of fractional change in capacitance for unit voltage at some DC voltage. In a MOS or SOS capacitor, the value of capacitance is given by a series combination of the oxide and space charge capacitances. For example, in a metal-oxide-silicon interface capacitor, there is only one space charge capacitance which is due to the depletion region formed in the silicon. However, in a poly-oxidepoly capacitor (SOS), there is a depletion region on both sides of the oxide, these depletion regions change as a function of voltage. The relationship of the capacitance value to the applied voltage coefficient for MOS capacitors is described in J. L. McCreary, "Matching Properties and Voltage and Temperature Dependence of MOS Capacitors", IEEE J. of Solid State Circuits, Vol. SC16, No. 6 (December 1981), pages 608-615.
In McCreary, it is noted that there is a partial cancellation of voltage coefficients for poly-oxide-poly or poly-oxide-silicon capacitors where the doping concentrations are approximately equal. However, the voltage coefficient is still significant in MOS capacitors utilizing one silicon interface or SOS capacitors utilizing two silicon interfaces with uneven doping due to processing variations. This is even true in the poly-oxide-poly capacitors wherein processing variations may result in different doping levels in the two plates at the silicon-to-oxide interface and there also may be doping gradients which result in differing dopant levels at the interface. These doping levels significantly affect the size of the depletion region and, subsequently, the voltage coefficient. This can be a problem during fabrication of the capacitors for the switched input of a delta-sigma type analog modulator in that cancellation of the first order voltage coefficient of the capacitor is realized only when the doping profiles are essentially identical in the two plates on poly-oxide-poly or poly-oxide-silicon capacitors. However, for practical processing these doping profiles tend to be different.